The present invention is directed to selectively dry etching a material comprising silicon dioxide over silicon. In particular, the present invention is directed to selectively dry etching phosphorous silicate glass (PSG) over crystalline silicon (doped or undoped) in the manufacture of photovoltaic solar cells.
PSG is formed in solar cell processing during emitter diffusion by exposing a crystalline silicon substrate to phosphorous oxychloride (POCl3) gas. Under an oxygen atmosphere phosphorous is driven into the silicon to form the n+ emitter of the solar cell. After the phosphorous diffusion process, the PSG is removed. Prior art processes for PSG removal typically employ wet chemicals such as, for example, hydrofluoric acid (HF), which are extremely harmful to environment and to the handlers.
Dry plasma etching processes have also been developed for PSG removal that do not suffer from the drawbacks of wet chemical processes. Such processes typically rely on fluorocarbon gases that, in the plasma state, form a polymer layer on the surface. Selectivity between the PSG and silicon is mainly attributed to the formation of this polymer layer because its growth on silicon surfaces is much faster thereby preventing further etching of the silicon. Such prior art dry plasma processes typically employ oxygen in the plasma to limit the amount of polymer formation. Oxygen plasmas, however, are problematic for several reasons. For example, in semiconductor applications, oxygen plasmas are known to damage the dielectric properties of low dielectric materials. Moreover, in solar cell applications where a PSG layer is being etched from a layer of phosphorous doped silicon, oxygen plasma tends to form SiO2 on the doped silicon surface which acts as an insulator by hindering the flow of electrons through the layer. Accordingly, there is a need in the art for a process for selectively etching a material comprising SiO2 over silicon that does not suffer from the above-mentioned drawbacks.